Assign index.downsample.interval setting when downsample index gets created. (#105241)

This avoids keeping downsamplingInterval field around. Additionally, the
downsample interval is known when downsample interval is invoked and
doesn't change.

server/src/main/java/org/elasticsearch/cluster/metadata/IndexMetadata.java

@@ -1276,8 +1276,7 @@ public class IndexMetadata implements Diffable<IndexMetadata>, ToXContentFragmen
         INDEX_DOWNSAMPLE_INTERVAL_KEY,
         "",
         Property.IndexScope,
-        Property.InternalIndex,
-        Property.PrivateIndex
+        Property.InternalIndex
     );
 
     // LIFECYCLE_NAME is here an as optimization, see LifecycleSettings.LIFECYCLE_NAME and

x-pack/plugin/downsample/src/main/java/org/elasticsearch/xpack/downsample/TransportDownsampleAction.java

@@ -115,7 +115,6 @@ public class TransportDownsampleAction extends AcknowledgedTransportMasterNodeAc
     private final IndexScopedSettings indexScopedSettings;
     private final ThreadContext threadContext;
     private final PersistentTasksService persistentTasksService;
-    private String downsamplingInterval;
 
     private static final Set<String> FORBIDDEN_SETTINGS = Set.of(
         IndexSettings.DEFAULT_PIPELINE.getKey(),
@@ -184,7 +183,6 @@ public class TransportDownsampleAction extends AcknowledgedTransportMasterNodeAc
         ActionListener<AcknowledgedResponse> listener
     ) {
         String sourceIndexName = request.getSourceIndex();
-        downsamplingInterval = request.getDownsampleConfig().getInterval().toString();
 
         final IndicesAccessControl indicesAccessControl = threadContext.getTransient(AuthorizationServiceField.INDICES_PERMISSIONS_KEY);
         if (indicesAccessControl != null) {
@@ -775,12 +773,14 @@ public class TransportDownsampleAction extends AcknowledgedTransportMasterNodeAc
          * case downsample will fail.
          */
         int numberOfReplicas = settings.getAsInt(Downsample.DOWNSAMPLE_MIN_NUMBER_OF_REPLICAS_NAME, 0);
+        var downsampleInterval = request.getDownsampleConfig().getInterval().toString();
         Settings.Builder builder = Settings.builder()
             .put(IndexMetadata.SETTING_INDEX_HIDDEN, true)
             .put(IndexMetadata.SETTING_NUMBER_OF_SHARDS, sourceIndexMetadata.getNumberOfShards())
             .put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, String.valueOf(numberOfReplicas))
             .put(IndexSettings.INDEX_REFRESH_INTERVAL_SETTING.getKey(), "-1")
-            .put(IndexMetadata.INDEX_DOWNSAMPLE_STATUS.getKey(), DownsampleTaskStatus.STARTED);
+            .put(IndexMetadata.INDEX_DOWNSAMPLE_STATUS.getKey(), DownsampleTaskStatus.STARTED)
+            .put(IndexMetadata.INDEX_DOWNSAMPLE_INTERVAL.getKey(), downsampleInterval);
         if (sourceIndexMetadata.getSettings().hasValue(MapperService.INDEX_MAPPING_TOTAL_FIELDS_LIMIT_SETTING.getKey())) {
             builder.put(
                 MapperService.INDEX_MAPPING_TOTAL_FIELDS_LIMIT_SETTING.getKey(),
@@ -910,7 +910,6 @@ public class TransportDownsampleAction extends AcknowledgedTransportMasterNodeAc
                             Settings.builder()
                                 .put(downsampleIndex.getSettings())
                                 .put(IndexMetadata.INDEX_DOWNSAMPLE_STATUS.getKey(), DownsampleTaskStatus.SUCCESS)
-                                .put(IndexMetadata.INDEX_DOWNSAMPLE_INTERVAL.getKey(), downsamplingInterval)
                                 .build(),
                             downsampleIndexName
                         );